Numerous methods are known for the production of polyaniline, including chemical, electrochemical, photochemical and enzymatic methods, via the use of templating agents, by means of plasma2. The chemical polymerisation methods in turn comprise heterophasic polymerisation, polymerisation in solution, interface polymerisation, seeded polymerisation, polymerisation via metathesis, self-assembly polymerisation and polymerisation via the use of ultrasounds. Usually the starting monomer is aniline and the first stage of polymerisation in acid solution is oxidisation with the formation of a cationic radical. As illustrated in FIG. 1, the cationic radical can be located either on the nitrogen atom or on the carbon atoms in ortho or para position (Reaction (1)). The radicals react with one another to form an aniline dimer, N-phenyl-1,4-phenylenediamine (DANI)6,7 (Reaction (2)).
Further oxidisation of the dimer generates radicals which combine to form oligomers of the aniline and, lastly, polyaniline itself (Reactions (3) and (4)). The dimer is a different chemical species which has a lower oxidisation potential and oxidises preferentially to the initial aniline. Chemical polymerisation is therefore an autocatalytic process, the kinetically limiting stage of which is the formation of the dimer8.
To improve its electric conductivity or vary its chemical and electrochemical properties, the polyaniline itself can be combined with graphene or reduced graphene oxide (rGO). Graphene is a structure composed of atoms of carbon arranged on a plane at the vertexes of hexagons. Graphene oxide (GO) has the same planar structure but with functional groups containing oxygen at the edges of the plane and included in the planar structure FIG. 210. Graphene oxide is obtained by exfoliation of graphite oxide10. The rGO has a structure similar to graphene but still retains some functional groups containing oxygen and is obtained by reduction of the graphene oxide. The rGO can be produced by chemical reduction, heat treatment or photoreduction11,12,13,14,15. The main reagents used to obtain rGO by means of chemical reduction are strong reducing agents like hydrazine, dimethylhydrazine, hydroquinone and sodium borohydride.
The methods for obtaining composites of PANI and rGO include direct mixing of PANT with rGO or reduction of graphene oxide (GO) in situ simultaneous with polymerisation in situ of the aniline in solution in the presence of rGO or GO18,19.
GO is a strong oxidant20 and for this reason has been used in the preparation of PANI-rGO composites in the form of nanofibres, exploiting the GO reduction process in the process of oxidative polymerisation of the aniline into polyaniline21. In this case the method of obtainment provides for the use of aniline as the starting monomer. Disadvantageously, aniline, a toxic substance and suspected carcinogen, significantly limits the application of said method. Furthermore the method described above leads to the formation of by-products bonded to the different types of radicals which the aniline can form in the first phase of polymerisation (Reaction (1) of FIG. 1), or a possible addition in ortho with formation of the dimer 2-Aminodiphenylamine thus producing a low regularity or branched polymer.
Furthermore the polyaniline/rGO composites obtained from aniline have shown that they do not form stable solutions or suspensions: the graphene does not remain in solution but can be easily separated by dissolution of the polyaniline into N,N-dimethylformamide and precipitation by centrifugation with partial reconstruction of the graphitic structure, as can be seen from X-ray analyses21.
Lastly, the reaction takes place slowly during a 24-hour period, with kinetics demonstrated by the thermogravimetric analysis tests21.
The need is therefore felt in the art for a preparation method for preparing composites of polyaniline and reduced graphene oxide from low toxicity reagents, which is conducted in an aqueous medium, which is quicker and simpler than the known methods, which allows a product to be obtained with high regularity and allows the preparation of stable polyaniline/reduced graphene solutions for applications such as flexible electronics obtained by inkjet printing.